Transglutaminases form isodipeptide crosslinks between acceptor amide groups of glutaminyl residues and donor epsilon-NH2 groups of lysines. In the epidermis, and other stratified squamous epithelial tissues, these enzymes are thought to be involved in the crosslinking of putative protein components to form the insoluble cell envelope. Using molecular biology approaches, we have found that there are 3 different transglutaminase activities in normal human and mouse epidermis. These are known as the K (TGase1), C (TGase2) and E (Tgase3) enzymes. We have isolated and characterized cDNA clones encoding full-length Tgase1 and Tgase3 systems. The complete genomic structure (14.1kbp) of the TGase1 system has been completed. Work is in progress on the genomic structure of the Tgase3 gene. Full-length and deletion constructs of the TGase1 enzyme have been expressed in E. coli and in mammalian cells in an effort to understand the structural/functional domains of this enzyme. The aim of all of these studies is to determine the likely functions of these different activities in normal epidermis, and whether or how these may be involved in pathology.